In many industrial applications, Systems Control And Data Acquisition (SCADA) may be performed through a number of deployed field devices. The field devices may be used for measuring such parameters as temperature, pressure, tank level, flow rate, valve state (e.g. open/closed) etc. or may be provided as control devices, e.g. valves. The field devices provide analog or digital signals to a Programmable Logic Controller (PLC) that provides the control logic for operating and monitoring an industrial process. The PLC may be provided at a central office such as within a PLC cabinet.
Communication between the field devices, in particular analog devices, and the PLC is typically provided using a bus network architecture operating a proprietary protocol. However, in order to enable data exchange between field devices and controllers of various manufacturers, a number of standard protocols were developed of which the Highway Addressable Remote Transducer (HART) protocol is just one example.
Traditional bus network architectures are limited by the number of nodes that can be supported. Large plant installations must therefore deploy numerous networks of field devices, with each network requiring a connection to an I/O module on a PLC. Thus for a large process plant or network of field devices, a number of PLCs may be required making integration of a complete system difficult. Technological advances as well as market demands for greater data acquisition and analysis are driving demands for higher bandwidth, greater throughput, faster data acquisition and faster information analysis than can be delivered by traditional bus network architectures, all at lower cost.
Industrial Ethernet standards are being developed to meet these demands with Ethernet gaining traction as the preferred factory network protocol because of its cost and performance benefits, hardware availability and ease of implementation. One such industrial Ethernet standard is IEEE 802.3af which has been developed as a standard for providing both communications and power through a single Ethernet cable. The system, termed Power over Ethernet (PoE) provides power and data through a single Ethernet cable, thereby reducing the cabling requirements for installations.
A problem that exists with implementing industrial Ethernet is that many existing field devices, in particular analog devices, are not configured for Ethernet communications. US Patent Application Serial No. 2007/0019560 (hereinafter Brewer) describes a system for communicating with field devices using PoE standards. Brewer appears to teach a system in which an interface module communicates by Ethernet protocols with a central office but by standard industrial protocols, e.g. HART, FieldBUS etc. on a bus architecture with field devices. The interface module appears to provide an intermediate device between an Ethernet enabled PLC or workstation and the bus network. Though the limitations of a bus network architecture are pushed further towards the outer edges of the network, the size, cost, speed and data limitations are still apparent, with any one interface module being limited to communication with up to four field devices and with the available power being shared across those four devices and the interface module.